The present invention relates to an objective lens for recording and/or reproducing of information for an optical information recording medium, a light-converging optical system, an optical pickup device, and a recording/reproducing apparatus.
With practical application of a red semiconductor laser, there has been developed and commercialized, in recent years, a DVD (digital versatile disc) that is a high density optical disc which is mostly the same as a CD (compact disc) representing a conventional optical disk (hereinafter referred to also as “optical information recording medium”) in size and has a large capacity. It is presumed that an advanced optical disc of a higher density type will appear in the near future. In an optical system of an optical information recording/reproducing apparatus wherein this advanced optical disc serves as a medium, a diameter of a spot converged on the information recording surface through an objective lens is required to be small, for achieving high density of recording signals or for reproducing high density recording signals. For that purpose, a shorter wavelength is requested for a laser representing a light source, and a greater numerical aperture is requested for an objective lens. What is expected as a short wavelength laser light source in terms of practical application is a violet semiconductor laser having an oscillation wavelength of about 400 nm.
Further, as an objective lens with high NA of NA 0.65 or more, there has been proposed a lens which is composed of two positive lenses to allot refracting power to four surfaces so that a radius of curvature of each surface may be large and sensitivity for error in injection molding and in lens forming may be eased accordingly. However, when NA is large like this, and when an objective lens with a small diameter is composed of two positive lenses, a working distance tends to be small, and there is a high possibility that the objective lens is brought into contact with an optical information recording medium by its warp, which is a problem.
Incidentally, when a shorter wavelength is attained for a laser light source and a greater numerical aperture is attained for an objective lens, it is presumed that there will be actualized problems which have been ignored in an optical pickup apparatus which conducts recording or reproducing of information for a conventional optical disc such as a CD and a DVD and is composed of a combination of a laser light source with a relatively long wavelength and an objective lens with a low numerical aperture.
One of the problems is a problem of longitudinal chromatic aberration caused on an objective lens by fluctuations of a fine oscillation wavelength of the laser light source. Since a wavelength of a light flux emitted from a semiconductor laser used as a light source in an optical pickup apparatus is monochromatic in general, the longitudinal chromatic aberration is considered not to be caused on an objective lens. However, there sometimes are occasions where there are caused mode hop phenomena in which a wavelength is changed instantly by several nanometers by a change of output. When the longitudinal chromatic aberration on the objective lens is not corrected, there is a possibility that a light-converging position is changed by the mode hop phenomenon and an error is caused on recording and/or reproducing of information. Since an amount of a change in light-converging position turns out to be greater as a wavelength of a light source becomes shorter, it is necessary to correct longitudinal chromatic aberration caused on the objective lens, when a short wavelength semiconductor laser with oscillation wavelength of 600 nm or less is used, and when a violet semiconductor laser with oscillation wavelength of about 400 nm is used in particular, as a light source of an optical pickup device.
Further, compared with a glass lens, a plastic lens shows greater changes for a refractive index and a form caused by a temperature change, and thereby, deterioration in power of the plastic lens caused by the changes in a refractive index and a form tends to be a problem. Since this deterioration in power, that is, an increase of spherical aberration is greater when NA is greater (deterioration in power is increased in proportion to fourth power of NA in general), there is a possibility that recording and/or reproducing of information may be interfered, if an objective lens made of plastic material of NA 0.65 or more is subjected to temperature changes of about ±30° C.
Further, another problem actualized in a shorter wavelength of a laser light source and a greater numerical aperture of an objective lens is a fluctuation of spherical aberration caused on an objective lens by fine fluctuations of oscillation wavelength of the light source. A semiconductor laser used as a light source in an optical pickup device has, on its oscillation wavelength, the scatter of about ±10 nm between individual bodies. When a semiconductor laser having an oscillation wavelength deviated from the standard wavelength is used as a light source, spherical aberration caused on an objective lens turns out to be greater as the numerical aperture becomes greater. Therefore, a semiconductor laser having an oscillation wavelength deviated from the standard wavelength cannot be used, and semiconductor lasers to be used as a light source need to be selected.
Further, another problem actualized in a shorter wavelength of a laser light source and a greater numerical aperture of an objective lens is a fluctuation of spherical aberration of an optical system caused by an error in a thickness of a protective layer (hereinafter referred to also as “transparent base board”) of an optical disc. Spherical aberration caused by an error of a thickness of a protective layer is generated in proportion to fourth power of the numerical aperture of an objective lens, and thereby, an influence of an error of a thickness of the protective layer grows greater as the numerical aperture of the objective lens becomes greater, and it is feared that stable recording or reproducing of information may not be carried out.